The readout is normally 'Gear Height' with the shocks un-compressed.
The reason it reads a negative value is because the rad - alt's themselves are forward of the gear on the body - and so a figure is applied to the height calculated to allow for height above gear at normal body angle on landing.
After landing this figure becomes negative because the figure is still applied. You occaisonally see it change on the ground as someone / something goes underneath it.